In our prior U.S. Pat. No. 3,942,846 there has been disclosed an assembly of this type in which the two relatively rotatable members, i.e. the leadscrew and the nut, are separated by a set of identical bearing balls recirculated via an endless track. The track is formed by aligned helical guide grooves on the confronting surfaces of the two members and by a return channel of greater depth in the body of the nut. The grooves have a depth equal to almost half the ball diameter and a width slightly smaller than that diameter whereby the balls are supported along bearing lines spaced from the groove bottoms. Such a two-point support, with positive guidance of the balls, can also be had with grooves of triangular, trapezoidal or ogival rather than arcuate cross-section. For a long service life with maintenance of the groove profile and pitch it is necessary that the members be made of steel which is hardened from their confronting surfaces down to at least the groove bottoms.
In the conventional manufacture of such a leadscrew, a correspondingly dimensioned steel rod is placed on a lathe in an unhardened state for the cutting of its track-forming guide groove. Since the subsequent hardening of the grooved rod invariably results in some deformation, the groove-cutting step is preceded by a trail run in which a substantially identical workpiece is machined and hardened to determine the extent of that deformation so that allowance can be made therefor in determining the pitch of the actual guide groove. Thereafter, the grooved rod is subjected to a hardening process, followed by tempering, cleansing and testing. Generally, the hardened leadscrew then requires some straightening and deburring as well as final grinding and honing of the groove profile.
The discontinuous surface of the grooved leadscrew creates some problems in the hardening step inasmuch as thermal stresses may produce cracks or weak spots. Also, uneven hardening due to temperature differences at the various levels may result in a creeping deformation of the workpiece so that the desired precision of the groove, profile and pitch cannot be achieved.
Similar inconveniences occur in the machining of the nut, albeit to a lesser degree in view of the reduced axial length of this member.